SWAPNA S. GOKHALE1, JOHN R. CRIGLER2, WILLIAM H. FARR2

Abstract:

Model-based analysis is commonly used to assess the influence of different factors on system availability. Most of the availability models reported in the literature consider the impact of redundancy, fault tolerance, and system structure. However, these models treat all system failures to be equivalent or at the same level of severity. In practice, it is well-known that failures are classified into multiple severity levels according to their impact on the system's ability to deliver its services. System availability is thus influenced by only some rather than all failures. To obtain an accurate availability estimate it is then necessary to incorporate failure severities into the analysis. In this paper we present a system availability model which considers failure severities of the hardware and software components of the system in an integrated manner. Based on the model we obtain closed form expressions which relate system availability to the failure and repair parameters of the hardware and software components comprising the system. For a given choice of failure parameters, we discuss how the closed form expressions could be used to select the repair parameters to achieve specified target system availability and to establish bounds on system availability. We illustrate the potential of the model by applying it to the failure data collected during the acceptance testing of a satellite system.